Have scientists discovered Earth's twin?

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MOUNTAIN VIEW, Calif. — NASA's planet-hunting Kepler spacecraft has confirmed the discovery of its first alien world in its host star's habitable zone — that just-right range of distances that could allow liquid water to exist — and found more than 1,000 new exoplanet candidates, researchers announced Monday.

The new finds bring the Kepler space telescope's total haul to 2,326 potential planets in its first 16 months of operation. These discoveries, if confirmed, would quadruple the current tally of worlds known to exist beyond our solar system, which recently topped 700.

The potentially habitable alien world, a first for Kepler, orbits a star very much like our own sun. The discovery brings scientists one step closer to finding a planet like our own — one that could conceivably harbor life, scientists said.

 NASA / Ames / JPL-Caltech

This diagram compares our own solar system to Kepler-22, a star system containing the first "habitable zone" planet discovered by NASA's Kepler mission.

"We're getting closer and closer to discovering the so-called 'Goldilocks planet,'" Pete Worden, director of NASA's Ames Research Center in Moffett Field, Calif., said during a news conference on Monday.

Hunting down alien planets The $600 million Kepler observatory launched in March 2009 to hunt for Earth-size alien planets in the habitable zone of their parent stars, where liquid water, and perhaps even life, might be able to exist.

Kepler detects alien planets using what's called the "transit method." It searches for tiny, telltale dips in a star's brightness caused when a planet transits — or crosses in front of — the star from Earth's perspective, blocking a fraction of the star's light.

The finds graduate from "candidates" to full-fledged planets after follow-up observations confirm that they're not false alarms. This process, which is usually done with large, ground-based telescopes, can take about a year.

The Kepler team released data from its first 13 months of operation back in February, announcing that the instrument had detected 1,235 planet candidates, including 54 in the habitable zone and 68 that are roughly Earth-size.

To date, just over two dozen of these potential exoplanets have been confirmed, but Kepler scientists have estimated that at least 80 percent of the instrument's discoveries should end up being the real deal. More >>